Solar cells are photovoltaic devices that can convert solar energy directly into electrical energy. Solar cells can be divided, according to the type of thin film material thereof, into inorganic solar cells and organic solar cells. A conventional solar cell is made of inorganic crystalline semiconductor material such as silicon (Si), which is doped to form a p-n junction. The electrons and holes created by the absorption of light diffuse to the p-n junction and are accelerated by the electric field to go to electrodes. The power conversion efficiency of this process is defined as the ratio of the electric power provided to the external circuit to the solar power incident on the solar cell. It has reached about 24%, as measured under standard stimulated conditions. However, conventional inorganic solar cells have limitations in terms of economic efficiency and material supply. For this reason, an organic semiconductor solar cell, which is easy to process and inexpensive and has various functions, is receiving attention as a long-term alternative energy source.
In the case of the organic solar cells, various organic semiconductor materials are used in small amounts, and thus material costs can be reduced. In addition, thin films can be formed by a wet process, and thus devices can be easily fabricated.
Meanwhile, it is important to increase the efficiency of solar cells so as to maximize the production of electrical energy from solar energy. In order to increase the efficiency of such solar cells, it is important not only to maximize the production of excitons in a semiconductor, but also to collect the produced charges without loss. One cause of the loss of electric charges is electric charge dissipation caused by the recombination of produced electrons and holes. Various methods for transferring produced electrons or holes to electrodes without loss have been suggested, but require additional processes that can increase the fabrication cost.
Organic solar cells were first introduced in the 1970s, but had no practical use because their efficiency was exceedingly low. However, in 1986, C. W. Tang (Eastman. Kodak) reported a two-layer structure comprising copper phthalocyanine (CuPc) and a perylene tetracarboxylic acid derivative, which could be put to practical use as solar cells, and since then, attention and research on organic solar cells have increased rapidly. In 1995, Yu. et al. introduced the concept of a bulk-heterojunction (BHJ). In addition, fulllerene derivatives having improved solubility, such as PCBM, have been developed for use as n-type semiconductor materials, resulting in a significant improvement in the efficiency of organic solar cells.